This invention relates to offshore platforms and, more particularly, to buoyancy units carried by the jackets for such platforms.
Offshore activities are being conducted with growing frequency in diverse scientific and industrial endeavors, such as ocean exploration and research, oil recovery, weather reporting, and radar defense, for example. In order to carry out these activities, large platforms or tower installations are being erected at offshore sites. Depending upon their location, such platforms can be on the order of two or three hundred feet or more in length. Consequently, very formidable problems exist in connection with platform fabrication as well as transportation and installation of the platform at its operating site.
Problems of this nature are discussed for example in U.S. Pat. Nos. 3,585,801 (Koehler -- June 22, 1971), 3,668,876 (Koehler -- June 13, 1972), 3,693,361 (Koehler -- Sept. 26, 1972), 3,823,564 (Crout et al -- July 16, 1974) and 3,859,804 (Koehler et al -- Jan. 14, 1975), all of which are assigned to the assignee of the present invention. The subject matter of these patents is incorporated herein by reference as if set forth at length. These patents discuss various techniques intended to overcome or alleviate fabrication, transporation and installation problems.
For instance, one accepted technique for installing a platform involves the floating of a prefabricated platform jacket to the operating site in a generally horizontal posture. (A "jacket" constitutes the basic platform framing which is usually connected at its lower end by piling to a submerged surface and which supports a deck structure at its upper end). Once at the site, the jacket is turned to an upright condition by appropriate ballasting of the tower and/or a transport structure, i.e., the bottom of the jacket is moved down through the water, so that the jacket assumes a generally upright posture. As is explained in Koehler et al U.S. Pat. No. 3,859,804 (Jan. 14, 1975), incorporated herein by reference, during this jacket installation procedure there may be a tendency for the jacket structure to roll about its own longitudinal axis or to make other potentially dangerous movements as it gains momentum while swinging through the water. It has been found that the effects of "roll" can be minimized by performing the installation operation in a relatively rapid manner. In so doing, however, it becomes necessary to maintain control over the jacket to prevent the buildup of great momentum which could endanger the safety of adjacent ships and personnel. In furtherance of this end, it has been proposed by Koehler et al that water-tight buoyancy spheres be attached to a pair of the jacket legs. The buoyancy spheres tend to brake the speed of the jacket as the latter swings through the water and control pitch and roll.
Once the jacket has been launched, ballasting of the buoyancy spheres and the jacket legs causes the jacket to descend in a generally vertical manner through the body of water. When the jacket has come to a rest on the waterbed, piling members are driven through piling guides carried by the jacket legs to pin the jacket to the waterbed.
While launching operations as outlined above have been successfully practiced, room for improvement remains. For example, it would be desirable to provide a jacket with pitch and roll resisting and possibly momentum-braking buoyancy equipment which could be employed with or in lieu of the previously described buoyancy spheres in a manner which would provide a compact or streamlined tower construction so as not to render the tower so unwieldy that it becomes more difficult to transport or launch. In addition, it would be useful to provide buoyancy equipment which applies a highly uniform transfer of forces between the jacket legs and the anchoring piles. Also, it would be advantageous for such buoyancy equipment to exert buoyancy forces without necessarily having to withstand large hydrostatic pressures that may be present. In other instances, where performance criteria require that such hydrostatic pressure forces be resisted, it would be desirable for the buoyancy equipment to be reinforced in an effective yet inexpensive and uncomplicated manner.
It is, therefore, an object of the present invention to overcome problems and achieve advantages set forth above.
It is another object of the invention to provide novel, more facile and economical methods and apparatus for exerting buoyant forces on an offshore tower such as a platform jacket.
It is a further object of the invention to provide such methods and apparatus which effectively controls tower motion during launching and installation operations.
It is another and independent object of the invention to provide such methods and apparatus which exerts buoyant forces on the tower absent the need for withstanding hydrostatic forces.
It is yet a further and independent object of the invention to provide apparatus which exerts buoyant forces on the tower while being effectively reinforced an sealed by way of a double-walled shell and pipe segment structure.
Still another independent object of the invention is to provide a tower buoyancy unit which utilizes a series of cells circumferentially disposed around a tower leg. The cells can be less than water-tight and filled with buoyant foam. Alternatively, the cells can be water-tight and include a novel double wall reinforced construction to resist hydrostatic forces and provide unique resistance to leakage.